Conventionally, high speed GaAs/AlGaAs delta doped quantum well field effect transistors have been manufactured by applying molecular beam epitaxy process (MBE). Japanese Patent Application No. Sho-61-276269 discloses one of such GaAs/AlGaAs delta-doped quantum well field effect transistors which are formed by applying molecular beam epitaxy process. In this prior art, a delta-doped quantum well is formed on a region other than the channel, and a doped GaAs is used on the gate. With such an arrangement, not only the break-down voltage is lowered, but no other superior characteristics can be obtained in the field effect transistor.
Another example of the prior art is the GaAs field effect transistor by A. Ishibashi et al, which is described in Electronics Letters, Volume 24, No. 16, pp1034, August 1988. In this GaAs field effect transistor which provides a gate length of 0.15 .mu.m, there is obtained a maximum intrinsic transconductance of 400 mS/mm which is a relatively high level.
A still another example of the prior art is a field effect transistor proposed by E. F. Schubert et al, which is described IEEE volume 33, No. 5, pp625, May 1986. According to this proposal, the molecular beam process is applied in such a manner that GaAs is doped in such a manner as to form an electronic concentration profile corresponding to Dirac delta function, thereby forming a field effect transistor. In this case, a high 2-dimensional electron gas (2 DEG), a high breakdown voltage and a high extrinsic transconductance can be obtained, thereby making it possible to obtain a high performance device. A still another case of the prior art is a field effect transistor by W. P. Hong et al, which is described in IEEE, volume 37, No. 8, pp1924-1926, August 1990.
The molecular beam epitaxy process is applied in forming this transistor, and a maximum transconductance of 160 mS/mm, a gate voltage swing of 2.1 V and a maximum drain current of 420 mA/mm are realized with a cap layer of 300 .ANG. and with a gate length of 1.3 .mu.m.
However, in the above described conventional techniques, the GaAs/AlGaAs quantum well field effect transistors are formed mostly by applying the molecular beam epitaxy process, and, if the molecular beam epitaxy process is to be carried out, an ultra high vacuum of below 10.sup.-11 torr has to be provided, with the result that the economy and productivity of the manufacturing process are aggravated.